The present invention relates to a method for manufacturing liquid crystal cell, and more particularly to a method for manufacturing bend-aligned liquid crystal cell using light, which is easier to manufacture than a conventional method using rubbing, and to a method for manufacturing bend-aligned liquid crystal cell with improved viewing angle.
A conventional liquid crystal display is mainly a twisted nematic liquid crystal display(referred as TNLCD) which has a character of changing a transmittance at each gray level according to the viewing angle. FIG. 1a is a graph showing a relationship between the transmittance and a voltage applied to said twisted nematic liquid crystal cell, FIG. 1b is a graph showing a relationship between the transmittance and a viewing angle at horizontal direction, and FIG. 1c is a graph showing a relationship between the transmittance and a viewing angle at vertical direction. Referring to FIG. 1a to 1c, while the transmittance is symmetrically distributed in the horizontal direction, the transmittance in the vertical direction is asymmetrically distributed. Therefore, because an image inversion occurs in the vertical direction, the viewing angle gets narrower.
To solve the problem, the multi-do-main twisted nematic liquid crystals are generally proposed. FIG. 2 is a view showing a structure of two-domain twisted nematic (TDTN) liquid crystal. In FIG. 2, each pixel has two direction configuration domains, where the two pretilted directions are in opposite directions. Applying a gray level voltage to this cell, the liquid crystal directors in two domain are tilted in opposite directions.
FIG. 3 is a view showing a structure of a domain-divided twisted nematic (DDTN) liquid crystal cell. In FIG. 3, the alignment layers 3,4 include two materials having different pretilt angles. The first alignment layer 3 is comprising an organic material, but the second alignment layer 4 is comprising an inorganic material so that direction of an average alignment angle on each alignment layer is in the opposite direction.
The two-domain TN LCD or domain-divided TN-LCD is manufactured by the following steps: rubbing the substrate coated on the alignment layer, inter alia, polyimide; masking some area with a photoresist; rubbing in the opposite direction reversely other area than said masking area; removing the photoresist. Because the process is too complicated as like above, cost is increased.
Generally, the cause of the narrow viewing angle is that the liquid crystal is caused by own character of liquid crystal as being an anisotropic material. Accordingly, a bend-aligned liquid crystal cell has been recently introduced. FIG. 4 is a view showing a structure of a bend-aligned liquid crystal cell, in which a bend-aligned liquid crystal cell 5 and an uniaxial film 6 with optical compensation by eliminating birefringence effect of said liquid crystal cell 5 are placed between two crossed(parallel) polarizer 7. A molecular of the bend-aligned liquid crystal cell 5, as shown in FIG. 4, maintains an acute angle at the surface alignment layer, but the angle becomes larger up to 90 degree at the midlayer between the alignment layers and, it becomes symmetrical structure near the substrate surface so that the viewing angle is improved. The bend-aligned of liquid crystal, having an angle of 90 degree between the molecules of liquid crystal and the alignment layer is formed by rubbing of alignment layer and by injecting liquid crystal maintaining previously decided pretilt angle. Further, if the electric field is applied to said bend-aligned liquid crystal cell, the transmittance is changed owing to the change of an effective refractive index caused by the change of the direction of liquid crystal by applying an electri field.
FIG. 5(a) is a view showing the relationship between the voltage of liquid crystal cell 5 and the transmittance, To is the point where the transmittance is 0, T.sub.50 is representing the point where the transmittance is 50, and T.sub.100 is representing the point where the transmittance is 100. FIG. 5(b) is a view showing the relationship between the transmittance and the viewing angle of the horizontal direction. The transmittance being originally 0, 50, and 100 respectively, is more symmetric than that of TNLCD in FIG. 1(b) depending upon the viewing angle. In addition, the transmittance of the vertical direction in FIG. 5(c) is always symmetric at T0, T50, and T100, while the image of the twisted nematic liquid crystal cell is inverted at the horizontal viewing angle of T50. Therefore, the viewing angle of a liquid crystal display using a bend-aligned liquid crystal cell 5 is highly improved in both the horizontal and the vertical direction than the twisted nematic liquid crystal cell.
However, there is still a problem of a particle and an electrostatic discharge on the alignment layer caused by rubbing process because the alignment layer in the bend-aligned liquid crystal cell 5 must be also rubbed before the injection of liquid crystal in order to form pretilt direction in the alignment layer. Particularly, the electrostatic discharge is the most serious problem in terms of the damage on the liquid crystal cell.